1. Field of the Invention
The invention relates to a molding composition based on a transparent polyamide for optical applications, the molding composition having a low level of intrinsic color together with a UV cut-off at about 400 nm. The invention further relates to the moldings produced from this molding composition.
2. Discussion of the Background
Transparent polyamides are used alongside other plastics, such as polycarbonate (PC) and polymethyl methacrylate (PMMA) as materials for producing spectacle frames, spectacle lenses, other lenses, magnifying glasses, sunglasses, and also for visors, protective masks, inspection windows, and protective screens. Amorphous polyamides are suitable for this purpose and are naturally transparent; polyamide materials which are transparent but microcrystalline have even better suitability, because when comparison is made with completely amorphous transparent polyamides, or else with PC or PMMA, they have increased scratch resistance and abrasion resistance, and also better chemicals resistance and stress cracking resistance resulting from the crystalline fraction.
In the case of optical applications such as the use of the transparent polyamides as lenses for sunglasses, the object is to inhibit the passage of UV light through the lens in order to avoid damage to the eye by light of these wavelengths. It is usual to attempt to achieve this object by introducing what are known as UV absorbers or UV stabilizers, and this also applies to transparent polyamide molding compositions.
For example, DE 196 42 885 A1 and U.S. Pat. No. 6,204,355 B1 mention the addition of UV stabilizers to transparent polyamide molding compositions. In these instances, however, the desired action of protecting the polymer appears to take precedence.
WO 02 090 421 A1 describes transparent thermoplastic polyamide molding compositions to which UV absorbers or UV stabilizers may be added in order to exclude UV light below 400 nm.
DE 37 17 928 A1 describes transparent polyamide molding compositions which may comprise optical brighteners, besides other ingredients. However, that specification does not say that the optical brightener is used to achieve a UV cut-off in the range from 370 to 430 nm.
UV cut-off here means that transmittance is less than 1% at a certain wavelength, the gradient of the transmittance curve at that point being positive. Once that point has been passed, the transmittance curve rises sharply.
The flank region of the UV/visible absorption spectra of the compounds used as UV absorbers or UV stabilizers exhibits a finite gradient. This means that there is no sudden fall-off of absorption from very high values to the value zero. If the corresponding transmittance curve is considered, this circumstance results in an S-shaped curve which rises with a finite gradient in the transition region between the value zero and the substance-specific maximum at a prescribed layer thickness. If the requirement is then to achieve exclusion of wavelengths below 400 nm in a transparent polyamide molding composition, using a commercially available UV absorber or UV stabilizer, the inevitable absorption of this compound at wavelengths above 400 nm results in marked yellowness of the molding composition. This is disadvantageous for the desired application. In the application in the lenses of sunglasses, for example, limitations arise with respect to the production of lenses with only slight tinting. Lenses with only slight tinting are advantageous in some applications, and increase the design freedom available to designers of sunglasses. In the case of slight tinting, a marked yellow perceived color of the molding composition remains discernable, and is unattractive. Any further addition of colorants in order to suppress this perceived color runs contrary to the initial objective of only slight tinting. In addition, a relatively high concentration of some colorants can bring about a loss of transparency.
If, in order to avoid these disadvantages, the UV absorber selected comprises one which, at the same concentration in the molding composition, has significant transmittance even at relatively low wavelengths, e.g. at 380 nm, it is sometimes possible to obtain a low-yellowness molding composition, but the UV protection then provided is inadequate.
Another factor which has to be considered is that the position of the transmittance curve for a molding composition is affected by the concentration of the absorber present. For example, addition of a relatively small amount of the same absorber generally leads to a shift of the cut-off toward smaller wavelengths, usually resulting in better color. However, the protective action provided is then also less than complete.
Yellowness can also result from thermo-oxidative degradation of the polymer.
As a measure of yellowness, use is made of what is known as the b value, which derives from the CIELAB color system (DIN 6174). A reduction in this b value within the positive range of values of b means a reduction in yellowness. Blueness occurs if the b value becomes negative.
It was therefore an object to provide a molding composition which better combines the property of exclusion of UV light with a minimum b value, this being a measure of yellowness of the molding composition, thus being superior to the molding compositions known hitherto in the simultaneous compliance with both criteria.